This invention will facilitate the aiming of Doppler flowmeters which are based on the art described by Hottinger (in U.S. Pat. No. 4,431,936), Fu et al (in U.S. Pat. Nos. 4,067,236 and 4,519,260), and Skidmore et al (in U.S. Pat. No. 4,807,636). The flowmeters disclosed in these patents measure volume blood flow in biological vessels, such as the aorta. Such systems are to be distinguished from the clinical pre-ejection period (PEP) monitor disclosed by Rapoport et al, in the IEEE TRANSACTIONS OF BIOMEDICAL ENGINEERING, vol. BME-26, No. 6, June 1979. The system disclosed by Rapoport et al was useful in following the changing location of a fetal heart by means of continuous wave (CW) measurements of Doppler signals. However, it was incapable of precisely centering a Doppler sensor on a blood vessel.
One important application of a Doppler flowmeter is the measurement of cardiac output of a human heart. Another important application of a Doppler flowmeter or velocimeter is the measurement of the velocity of the blood flow from a human heart. The relevant anatomical details are shown in FIGS. 1A and 1B, which are respectively anterior and sagittal views of a human torso. These measurements are accomplished by measuring the forward blood flow in the ascending aorta 20, which emanates from the left ventricle of the heart (not shown) and is typically circular in cross section.
As shown in FIGS. 1A and 1B, the suprasternal notch 22 provides an acoustic window for the measurement of the blood flow in the ascending aorta 20. Simple pulsed wave and continuous wave Doppler devices such as ultrasonic transducer 24 located in the suprasternal notch 22 have been used to measure the integral of the systolic velocity of forward moving blood in the aorta. As can be seen from FIGS. 1A and 1B, the transducer 24 projects a beam 26 of ultrasonic energy which projects downwardly, substantially along the axis of the ascending aorta 20. When these measurements are combined with echo image measures of the aortic cross-sectional area, cardiac output can be calculated.
Simple Doppler velocimeters have not been routinely adopted as cardiac output devices for several reasons. Foremost, they cannot provide accurate flow measurements when they are not used in conjunction with an imaging device. In addition, trained ultrasonic personnel and suitable imaging equipment are not routinely available.
Simple velocimeters have suffered from several other theoretical drawbacks in their role as cardiac output devices. First, the velocity measurements that they make underestimate the true lumenal velocity as a function of the cosine of the angle of incidence of the ultrasonic beam relative to flow. Second, most commercially available devices generate beams which are not wide enough to uniformly insonify the breadth of the aortic lumen, with the consequence that uncertainty exists as to the relationship between the measured and true mean lumenal velocity. Third, clinicians have had difficulty in unambiguously aligning the Doppler sample volume with the center of the aorta. This has been a particularly troublesome aspect of continuous wave (CW) velocimeters, which create an axially large sample volume and, consequently, can easily interrogate arteries other than the aorta (particularly the innominate artery) from the suprasternal notch.
Clinical research has recently demonstrated the accuracy of a non-invasive cardiac output device which is based upon the attenuation compensated volume flowmeter (ACVF) principle first described by Hottinger. See, for example, Determination of cardiac output in critically ill patients by dual beam doppler echocardiograph, JACC, v. 13, No. 2, pp. 340-37, 1989, by Looyenga et al.
This device represents a major advance over the previously available Doppler cardiac output technology in that its measurements of flow can be made without an imaging device and are angle independent. Furthermore, when properly implemented, devices based upon this principle of operation do not suffer from the potential measurement errors due to non-uniform insonification of the aorta.
The device employs the annular array beam forming technology described by Fu and Gertzberg in ULTRASOUND IMAGING, 5, pp. 1-16 (1983). The Hottinger principle calls for the simultaneous generation of two overlapping Doppler sample volumes. A narrow sample volume must reside wholly within moving blood, while a wider sample volume must uniformly insonify a cross-sectional slice of the relevant biological vessel.
FIG. 2 is a schematic view of the interrogation profile of an ACVF device aimed at the aorta 20. The transducer 24 produces wide and narrow ultrasonic beams 26A and 26B. It has been known in the prior art to measure the cardiac output of the heart 28 through the ascending aorta 20 from measurements of the mean velocity V and the cross-section area A. The Doppler signal derived from the wide beam 26A is used to obtain a mean velocity estimate, and the ratio of the Doppler power present in the wide and narrow beams 26A and 26B is used to obtain an estimate of the projected aortic area. Multiplication of the two terms yields the instantaneous flow rate.
The primary impediment to the widescale commercial acceptance of such devices is the practical difficulty clinical personnel have in aligning the Doppler sample volumes with the aortic lumen. The criticality of the alignment process is accentuated by the fact that for various reasons (see below), the wide sample volume is often just large enough to uniformly insonify large aortas and the narrow sample volume is often just small enough to fit within the smallest aortic lumens. To our knowledge, no reliable criteria have been developed which unambiguously inform clinicians that Hottinger-type flowmeters are properly aligned relative to the aorta.
It has been suggested that maximization of the Doppler power in the wide and narrow sample volumes can serve as an adequate signature that the beams are acceptably centered about the aorta. FIGS. 4A-C are schematic representations of the wide and narrow power received as a function of sample volume position relative to the aorta. A sample volume position search will invariably be undertaken when operating with an apparatus as shown in FIG. 2. FIGS. 4A-C show that maximum wide and narrow power are achieved without fulfillment of the Hottinger requirements for sample volume placement, namely, uniform insonification of the aorta by the wide beam and placement of the narrow beam wholly within moving blood. The figures demonstrate that unless the acoustic search is thorough enough to bring the sample volumes through the center of the aorta, maximization of the wide and narrow powers may not be adequate criteria for sample volume localization.
It has also been suggested that maximization of the velocity in the narrow beam will result in adequate centering of the ACVF sample volumes in the aorta. The following discussion regarding the complex nature of the aortic velocity profile will serve to point out the limitations inherent in this approach.
FIG. 3 is a view of the blood flow velocity profiles expected in an aorta 30 and the left ventricular outflow tract 31 when no dilation of the ascending aorta is present. The aorta 30 is defined by a wall 32. The blood velocity profile at station 34 at the aortic orifice 33 is indicated by the array of arrows 36 which show the direction and magnitude of the velocity of the blood as a function of transverse position at station 34. The fact that the blood velocity at station 34 is substantially constant, even very close to the wall 32, is indicated by the series of parallel, equal-length arrows 32. The blood velocity profile at station 38, which is located downstream from station 34, is indicated by arrows 40. At station 38, the blood velocities nearest the wall 32 are lower than those in the center of the aorta 30, and, at the beginning of the aortic arch 46, the velocity is maximum near the inner curvature of the aorta 30. This skewing effect is seen even more clearly at station 42, where the innermost of the arrows 44 indicates that the maximum blood velocity is nearest the inner portion of the wall 32.
Because the portion of the aorta 30 in the aortic arch 46 is curved in two dimensions, the actual skew of the velocity profile may be complex. In addition, the branching of the arteries 48, 50, and 52 in this area to the arms and the head influences the blood velocity profile, especially at station 54. Further, wave reflections from these arterial branches will affect the blood velocity profiles near this portion of the ascending aorta.
The velocity profile skew is expected to change during systole, due to the different effects of blood acceleration and deceleration on the velocity profile. Another factor which complicates the flow of blood through the aorta is the transverse movement of the blood which occurs as the aortic wall 32 expands during systole. Because of the complicated nature of the blood flow in the aorta, it is clear that simple estimates of cardiac output, based, for example, on maximum velocity at a particular aortic station, can be very inaccurate.
Given the variability in human anatomy, it is difficult to predict whether a preset Doppler sample depth will reside at a level closer to station 34 or 42. If the Doppler sample volume resides at or near level 34, it may easily prove that the velocity gradient across the lumen is too slight to adequately distinguish the center of the lumen by maximization of narrow beam velocity, with the consequence that the wide beam may not be adequately centered. If, on the other hand, the sample depth occurs at or near station 42, manipulation of the transducer to achieve maximization of the velocity in the narrow beam will clearly place the sample volume to the side of the lumen. This could result in uneven insonification of the aorta by the wide beam, and possibly failure to place the narrow beam wholly within moving blood.
The invention provides a practical means for providing directional alignment information and confirmation that the Doppler sample volumes are acceptably centered about the aortic lumen during ACVF measurements. The invention also provides a practical and general means for providing directional alignment information for Doppler velocimeters.